This invention relates to a process for producing a rod lens array in which a multiple of gradient index rod lenses are precision arranged parallel to each other at given spacings and fixed between two side plates (frames) as they are buried in a resin. The rod lens array is typically useful as an optical component for an image writing system in an electrophoto-graphic printer.
The rod lens array is an optical component that has a multiple of tiny gradient index rod lenses arranged in alignment so that they combine together to form a single continuous erected unit-magnification image. The rod lens array has a short optical pathlength and needs no inverting mirror; because of the size-reducing feature, the rod lens array is commonly used as an optical component not only for an image reading system in an image scanner, a copier, etc. but also for an image writing system that forms a latent image on a photoreceptor in accordance with supplied image signals. Recent models of electrophotographic printer have been improved to achieve high image resolution comparable to that offered by silver halide photography. This has accordingly increased the requirement for higher precision in latent image and, hence, for better quality of image writing optical components in terns of the precision of the position in which image is formed.
A common process for the production of the rod lens array includes an assembly step in which a multiple of lens performers are arranged in contact with each other and fixed between an upper and a lower side plate (frame plate) to form a block, an impregnation and curing step in which a resin is impregnated between the individual lens performers in the block, and a cutting step in which the block is cut to a specified lens length. The side plates are usually fiber glass-reinforced plastic laminated plates (FRP plates).
In the assembly step, two methods are commonly used. In one method, a plurality of gradient index rod lenses are aligned on a flat frame such that their outer peripheral surfaces contact each other and they are altogether fixed to maintain the aligned state (this method is hereunder referred to as the “diameter referenced method”). The other method uses an aligning tool having a plurality of V-shaped grooves cut side by side in the surface of a platen on specified pitches, and gradient index rod lenses placed in the respective V-shaped grooves are fixed altogether to maintain their aligned state (this method is hereunder referred to as the “mechanically referenced method”; see JP 9-90105 A).
The surface of an FRP plate has fine periodical asperities due to the texture of the woven glass fabric used in its production and hence the individual lens preformers are likely to be offset positionally during the assembly and impregnation steps. Positional offset also occurs due to the warpage of lens preformers and their surface roughness. In addition, both the diameter referenced method and the mechanically referenced method have their own problems.
Consider, for example, an image writing system of the type shown in FIG. 11; a light emitting portion (LED device) 10 blinks in accordance with image signals and a rod lens 12 forms a latent image on an imaging surface (photoreceptor drum) 14. Any deviation from the desired alignment of rod lenses 12 results in a great variation in the potential at which the latent image is formed on the photoreceptor drum. On account of this variation in the imaging position that results from misalignment of the rod lenses, the image resolution that can be achieved by the image writing optical system is limited.
To make an acceptable rod lens array, gradient index rod lenses must be aligned such that adjacent lenses have a constant axial spacing (or “alignment pitch”) and that there be neither inclination in the plane of alignment (which is hereunder referred to as “horizontal inclination”) nor inclination in a direction normal to the plane or alignment (which is hereunder referred to as “height inclination”). The height inclination can be suppressed in the diameter referenced method. On the other hand, due to contact between gradient index rod lenses, a horizontally inclined lens affects an adjacent lens and the lenses taken as a whole may occasionally be inclined horizontally to suffer “axial displacement”. In the case of a printer or facsimile, this causes an image to be formed in a position far distant from where it should be.
The mechanically referenced method can provide higher precision in lens alignment. On the other hand, individual gradient index rod lenses sometimes fail to be placed uniformly within V-shaped grooves in the platen, causing one lens to be inclined with respect to another. Since it is unavoidable that lens preformers vary in diameter, the setting of the pitch between grooves in the platen must not be smaller than a maximum value for the variation in the diameter of lens preformers. As a result, very small gaps occur between arranged lens preformers and positional offset may occur during the mounting of side plates. In the “partial” burial step where one side plate is mounted, the grooved platen adequately protects against positional offset but in the “complete” burial step where the grooved platen is removed and the other side plate is mounted, the precision in the alignment of lens preformers may drop since there is nothing that controls the positional offset. In a printer or facsimile, this is a cause of failures such as an overlap of pixels.
Next, we describe the positional offset of rod lenses and their departure from the desired alignment on account of their surface roughness. The rod lenses to be used in the rod lens array are mainly produced by ion-exchange. As shown graphically in FIG. 17, a beam of incident light which falls on an end face of a rod lens 12 at an angle smaller than its angular aperture θ0 is an effective ray 21. On the other hand, a beam of light incident at an angle greater than θ0 undergoes regular reflection at the internal specular surface of the rod lens 12 which is manufactured by drawing. The reflected beam is so-called “stray” light 22 which takes no part in image formation and therefore lowers the contrast of the rod lens 12. Furthermore, the rod lens array is constructed by a multiple of rod lenses 12 and the stray light 22 occurring in individual rod lenses 12 will reduce the overall contrast of the rod lens array.
In known rod lens arrays, a multiple of rod lenses arranged in one or two rows are fixed between two frame plates and a black silicone resin is filled between lenses and between each frame plate and lenses.
FIG. 18 shows a conventional method of removing stray light 22 by allowing it to scatter. To this end, the peripheral surface of the rod lens 12 is removed to some extent by a surface treatment and tiny asperities 23 are formed around it (see, for example, JP 58-38901 A). The stray light 22 incident on the surface of the area where asperities 23 are formed is scattered as indicated by 25. In addition, the black silicone resin 24 covering the peripheral surface of the rod lens 12 absorbs the scattered light 25, eventually suppressing the stray light 22.
In fact, however, the peripheral surface of the conventional rod lens has a profile as shown in FIG. 19 and suffers the following disadvantages. FIG. 19 shows the roughness, or the degree of unevenness of the peripheral surface of a rectilinear area in the longitudinal direction of the conventional rod lens.
The first problem arises from the fact that the amount of removal from the peripheral lens surface varies from one lens to another and so does the lens diameter. If the rod lenses are arranged with reference to the frame plates, there occurs a departure from the desired lens arrangement due to the variation in lens diameter and the optical axis of one rod lens is inclined with respect to the optical axis of another.
Secondly, due to the lens-to-lens difference in the roughness of the peripheral lens surface, there occurs a variation in the effective aperture of the lens that contributes to satisfactory imaging. This causes a variation in the resolving power of the rod lens array in a longitudinal direction. With decreasing surface asperities, the lens surface has an increased degree of specularity and becomes more susceptible to the effect of stray light; it is therefore considered necessary that the peripheral surface of each rod lens have a center-line-average roughness of at least 0.5 μm.